White House gun proposals and a look back at past mass shootings

If the White House gun violence policy announced today had been in effect, would it have changed the outcome of recent mass shootings?

It's impossible to say for certain, but here's a look at what it might have meant.

NEWTOWN -- A tough assault weapons ban might have blocked the sale of the Bushmaster XM-15 that Adam Lanza's mother bought legally and which police say he used to kill all his victims in the school. A ban on high-capacity magazines might also have reduced the killing power of the weapons he brought to the school.

Jason Reed / Reuters

President Barack Obama signs executive orders on gun violence during an event at the White House in Washington, January 16, 2013.

AURORA -- As with Newtown, a ban on assault weapons and high-capacity magazines might have reduced the killing power that James Holmes brought into the movie theater.

TUCSON -- Federal law bars gun sales to anyone who "is an unlawful user of or addicted to any controlled substance." When Jared Loughner attempted to enlist in the Army, he admitted he was a drug user. That information was never reported to the FBI for inclusion in the background check database, because a Justice Department policy dating from the Clinton administration directed federal agencies not to report information volunteered by drug users, for fear that it would deter drug users from seeking treatment.

We are waiting to hear from the administration on whether President Obama's executive actions announced today will rescind that Clinton-era policy.

VIRGINIA TECH -- The dealers who sold Seung Hui Cho the guns he used in the Virginia Tech shooting followed the law to the letter, because there was no information in the system indicating that he was not qualified to buy a firearm. Federal law bans gun sales to anyone found by a court to be a danger to himself or others because of mental illness, but under the law then in effect in Virginia, the state entered people like Cho into the gun check computers only if they'd actually been admitted as patients to a hospital. Virginia has since eliminated that loophole to bring its practices in line with federal law.

As for requiring universal background checks, two examples come to mind:

COLUMBINE -- Eric Harris and Dylan Klebold were both under age 18, the minimum age under federal law to buy a rifle or shotgun from a licensed gun dealer. They recruited an 18-year-old Columbine High School senior to help them buy several guns used in the killings. The three went to a gun show in Adams County, Colorado and bought a semiautomatic assault rifle and two shotguns with cash. Klebold and Harris also bought an assault pistol from a private seller who had bought it at the gun show. These were the four weapons used in the Columbine attack.

L.A. JEWISH COMMUNITY CENTER -- In 1999, Buford Furrow, a white supremacist, shot and killed a letter carrier then entered the LA Jewish Community center and shot five other people, including three children. He bought the gun, without a background check, at a gun show in Washington state. As a convicted felon, he could not have passed a federal background check.